1. Field of the Invention
This invention relates to nanostructured materials based on carbon nanofibres and/or nanotubes.
2. Description of the Related Art
Carbon nanofibres and nanotubes are materials which are well known in the state of the art. By “carbon nanofibre or nanotube” is meant for the purposes of this description a tubular carbon-based structure which is generally essentially based on carbon in the form of sheets of graphene having diameters of between 2 and 200 nm (dimensions which can be measured in particular from electron microscope images). These compounds belong to the family known as “nanostructured materials” which have at least one characteristic dimension of the order of a nanometer.
There are many types of carbon nanofibres and nanotubes of this kind, such as nanofibres comprising stacks of flat graphene sheets (known as “stacks”) or “cones”, or else carbon nanotubes, which are hollow cylindrical structures based on sheets of graphene rolled on themselves, a same nanotube often comprising several concentric cylinders based on graphene. These materials are generally obtained by progressive growth on metal catalysts in dispersed form, especially by the so-called process of “vapour deposition”, which comprises placing a gas containing a carbon source into contact with a catalyst based on a transition metal which is in the metallic state, in the powder state, or which is supported. For more details concerning these materials and their manner of synthesis reference may be made in particular to the articles “Nanotubes from carbon” by P. M. Ajayan (Chem. Rev., vol. 99, p. 1787, 1999) and “Carbon nanofibers: catalytic synthesis and applications” by K. de Jong and J. W. Geus (Catal. Rev. Sci. Eng., vol. 42, p. 481, 2000).
As a general rule, whatever their precise structure may be, nanostructured materials of the carbon nanofibres and nanotubes type have very useful physical and chemical properties.
Normally, carbon nanofibres and nanotubes generally have a relatively high specific surface area associated with good intrinsic mechanical strength, which makes them substrates of choice, especially for the deposition of catalytic species.
Furthermore, given their nanometric dimensions, carbon nanotubes and nanofibres very often have intrinsic properties which are useful in catalysis. In particular, it is known from the article “Mesoporous carbon nanotubes for use as support in catalysis and as nanosized reactors for one-dimensional inorganic material synthesis” (Appl. Catal. A, 254, 345, 2003), that carbon nanotubes behave as “nanoreactors”, their internal space defining a specific environment wherein the conditions under which chemical reactions take place are modified. In fact the internal diameter of a nanotube is generally of the order of 1 to 100 nm, which creates a space in which the conditions under which chemical species meet can be channeled and optimised. Thus, the use of nanotubes makes it possible to achieve high yields at ambient temperature and atmospheric pressure in reactions which under conventional conditions would require the use of higher temperatures and/or pressures. In this context, nanotubes can especially be used to catalyse Friedel-Crafts reactions, or again reactions for the desulphurisation of effluents containing H2S, for example the final desulphurisation of effluents from the petroleum industry. In that respect reference may be made to the article “Carbon nanotubes as nanosized reactor for the selective oxidation of H2S into elemental sulphur” (Catal. Today, 91-92, 91, 2004).
Nevertheless, apart from these various advantages, nanostructured compounds based on carbon nanotubes or nanofibres are very frequently of very small size, which makes them very difficult to handle and control.
Especially carbon nanotubes and nanofibres have a very strong tendency to dusting, which is particularly likely to give rise to environmental and safety problems, in view of the high reactivity of the carbon nanotubes and nanofibres.
In addition, it should further be noted that, because of their very small size, carbon nanotubes and nanofibres are generally not filterable. It is therefore difficult to decontaminate air containing them in dust form. It is also difficult to recover catalysts based on carbon nanotubes and nanotubes after they have been used in a liquid medium, for example at the end of reactions in reactors of the stirred bed type.
Moreover, the small size of carbon nanotubes and nanofibres also prevents their use in reactors of the fixed bed type, particularly because of problems of loss of head.
For all these reasons, in practice, carbon nanotubes and nanofibres find little employment per se as catalysts or supports for catalysts in industrial processes.
In order to overcome the aforesaid disadvantages of carbon nanofibres and nanotubes, namely the difficulty of shaping them, the dust to which they give rise and the difficulty of using them in reactors of the fixed or stirred bed type, one solution, which has been especially suggested in application WO 03/048039, comprises immobilising these nanofibres and nanotubes on supports, for example on beads, felts, fibres, foams, extrusions, solid blocks or pellets.
This solution is certainly useful, but it requires the use of a carrier substrate, which is generally inactive in terms of catalysis. Thus in materials of the type of those described in WO 03/048039, the mass of the carrier substrate may represent up to 90% of the total mass of the material, which reduces the overall effectiveness of the material in relation to its total mass. In addition to this phenomenon of diluting catalytic activity, the presence of the carrier substrate very often means that, in the case of nanotubes, one of the inlets of the tube is not available since it is connected to the carrier substrate. Therefore, only one of the inlet to each of the nanotubes is available, which limits the catalytic efficiency of each nanotube and therefore again reduces the overall catalytic efficiency of the material.